Process for anodizing magnesium and magnesium alloys



United States Patent O1 ice 3,477,921 Patented Nov. 11, 1969 3,477,921 PROCESS FOR ANODIZING MAGNESIUM AND MAGNESIUM ALLOYS Terry Lee Sanford, Fullerton, Calif. (419 N. Pauline St., Anaheim, Calif. 92805) No Drawing. Filed Apr. 10, 1967, Ser. No. 635,289 Int. Cl. C23b 9/ 06' US. Cl. 204-56 12 Claims ABSTRACT OF THE DISCLOSURE A process for applying improved corrosion and abrasion resistant coatings on magnesium and magnesium alloys by anodic oxidation in an aqueous solution of boron compounds and chlorides. Said coatings which contain the minerals boracite and enstatite are more dense and glassy than those produced heretofore.

This invention relates to electrolytes, and is particu larly concerned with a composition of chemicals and with electolytes containing such chemicals, and which are especially useful for the production of hard wear and corrosion-resistant metallic oxides and chlorides, including boracite and enstatite, as films or coatings on magnesium and magnesium alloys by electrolytic oxidation and chlorination of magnesium by employing such electrolytes.

Mganesium in its pure or alloyed state is a soft metal and extremely susceptible to corrosive action, therefore an attempt has been made in the aircraft, missile and other industries to give the metal a protective coating against corrosion and wear. Processes already known to industry have generally been limited to chromate pickles, fluorides and/or phosphate coatings which are softer than the parent metal and provide inadequate protection against either corrosive action or wear. Another limiting factor of known protective films is their tendency to spall off under pressure or bending, permitting rapid corrosion and pitting in the area thereby left unprotected.

The process of my invention employing the chemical electrolyte(s), herein described, is capable of making a denser and more glassy coating, including the minerals boracite and enstatite, providing greater wear and corrosion protection than any films of prior art known to me, and also produces mineral coatings which prior art processes are incapable of producing. Also the coating is deposited more rapidly than heretofore, with better bonding to the metal than prior art.

An object of this invention is to afford procedure for applying a tough, hard coating of boracite on magnesium and its alloys by electrolytic oxidation and chlorination employing an aqueous solution of the chemicals borax and/or potassium borate, boric acid, hydrochloric acid and/or barium chloride as electrolyte(s).

Another object is to afford procedures for applying a further coating of enstatite (MgSiO to the coating of boracite, or as a separate coating, by means of a solution and/or a suspension of finely ground bentonite or other clay in the electrolytic solution described above.

Still another object of this invention is to afford procedures for applying a tough, hard coating that will resist wear and corrosion on parts made of magnesium and its alloys by electrolytically depositing a filler of magnesium oxide in the pores of the chloride or silicate coating, if pores occur.

Other objects and advantages of the invention will be apparent from the following description of my invention. I have obtained hard, tough coatings of magnesium chloride silicate and oxide, and/or boron oxides, on magnesium and its alloys by employing all, or various combinations of borax, potassium borate, boric acid, hydrochloric acid, barium chloride and bentonite, which coatings are wear resistant and more protective against corrosive action on the parent metal than coatings known to prior art.

The electrolytic oxidation, chlorination and/or silicate formations are carried out according to my invention by cleaning the magnesium, or alloy, part with chemical or abrasive means known in the plating or metal coating industry, then making the metal part the anode of an electrolytic cell which may be in the form of a stainless steel. tank, which acts as the cathode, the cell containing an aqueous solution of the several chemicals named above. Sul'ficient voltage and current density is applied to the cell for a period of l to 60 minutes depending on the thickness of the coating desired. The coated magnesium, or alloy, part is then removed from the electrolyte, rinsed in clean water and air dried.

Thus I may use a solution of from 0.1% by weight up to the saturation point (at temperatures of 50 to F.) in water of each of the chemicals; boric acid, borax, potassium borate and barium chloride, adjusting the pH of the solution between pH 5 and pH 9 by the proper addition of hydrochloric acid or borax. I generally stir into the aqueous solution from 0.1 to 1.0% by weight of finely ground bentonite or clay. It is understood that these proportions .are not critical and may be varied. I may operate my process at any temperature above freezing of the solution up to F. The following are examples illustrating the mixing of the required chemicals and the application of such solutions in my electrolytic processes.

EXAMPLE 1 An aqueous solution containing 3 grams of sodium tetraborate (Na B O or potassium borate (K B O per liter of H 0 is adjusted to pH 6-pH 7 by the addition of about 1.5 ml. per liter of solution of concentrated hydrochloric acid (HCl). A cleaned magnesium alloy part is suspended in this solution as an electrolyte and made the anode with the stainless steel cell the cathode. A direct current of 5 volts, which creates a current density of 0.5 ampere, is applied to the part for 4 minutes, the part is withdrawn from the electrolyte with the current still on and the part is rinsed in clean water and air dried by draining. A coating of approximately 0.0001 inch has been applied to the surface of the part by this process and when tested in a salt spray (fog) chamber according to MILSTD-141A, Method 6061, the coating has been found to protect the part from corrosion for two to three (or more) times as long as the same alloy parts coated by prior art.

EXAMPLE 2 EXAMPLE 3 An aqueous solution of 4 grams of barium chloride (BaCl per liter of water is adjusted to pH 5.5 to pH 6.5 by the addition of boric acid (H BO and is used as the electrolyte for coating 2. magnesium alloy part as described in Example 1. A clear coating of approximately 0.0001 in thickness is thereby applied to the part by this process which is resistant to corrosive action of atmosphere and salt fog. Other metallic chlorides may be substituted for barium chloride with various formations of the protective (borate) coating.

EXAMPLE 4 To the aqueous solution described in Example 3 I add 1.0 gram per liter of solution of finely ground bentonite, or other clay, and a magnesium alloy part is electrolytically coated as described in Examples 2 and 3.

EXAMPLE 5 An aqueous solution of 4 grams of barium chloride per liter of water is used as the electrolyte for coating a magnesium alloy part as described in Example 2, and a tough corrosion resistant coating is produced on the magnesium alloy part.

While I have described a particular embodiment of my invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

I claim:

1. A novel process for the production of a hard, wear resistant, corrosion resistant, anodic coating on magnesium and magnesium alloys comprising anodizing said metals in an aqueous solution of boric acid or a boron salt and hydrochloric acid or a chloride.

2. A process as defined in claim 1 in which said aqueous solution comprises sodium tetraborate and hydrochloric acid.

3. A process as defined in claim 1 in which said aqueous solution comprises potassium metaborate and hydrochloric acid.

4. A process as defined in claim 2 in which said aqueous solution comprises sodium tetraborate, hydrochloric acid and a suspension of clay.

5. A process for producing a hard coating on magnesium or magnesium alloys anodically, in which the hard coating contains boracite.

6. A process as defined in claim 1 in which said aqueous solution comprises barium chloride and boric acid.

7. A process as defined in claim 1 in which said aque- 4 ous solution comprises barium chloride and a suspension of bentonite or clay.

8. A process for coatingmagnesium and magnesium alloys with a hard, tough coating which comprises passing a gradually increasing current of electricity of from 2 to 6 amperes per square foot of the area of the part being anodized through an electric cell comprising a solution of sodium tetraborate or potassium borate in a concentration of from 3 grams per liter of distilled water up to the saturation point of the borate, with the solution adjusted to the pH of 5 to pH of 7.5 by adding hyrochloric acid, :and continuing the anodizing process for from 3 to 16 minutes with the magnesium (or alloy) part the anode and the stainless steel tank the cathode.

9. A process as defined in claim 8 wherein the solution is maintained at a temperature between the freezing point of the solution and F.

10. A process as defined in claim 9 wherein the solution contains from 1.0 to 30 grams per liter of clay held in suspension by agitation of the solution.

11. A process as defined in claim 8 wherein the aqueous solution contains barium chloride in a concentration of from 3 grams per liter of water up to the saturation point of the salt and boric acid is added to maintain the pH between a pH of 5 and a pH of 7.5.

12. A process as defined in claim 8 wherein the solution contains potassium metaborate in a concentration of from 3 grams per liter up to the saturation point of the salt and the pH of 5 to pH of 7.5 is maintained by additions of hyrochloric acid, with finely divided clay held in suspension by agitation of the solution and the temperature of the solution is maintained between the freezing point of the solution and 95 F.

References Cited UNITED STATES PATENTS 8/1959 De Long 20456 2/1960 Ellwood et al. 204-56 

